1. Field of the Invention
The present invention relates to a semiconductor integrated circuit and an optical transmitter module, and more particularly, to a semiconductor integrated circuit capable of increasing the gain and band of an amplifier constructed from a field-effect transistor (PET).
2. Description of the Related Art
Recently, driver ICs (semiconductor integrated circuits) used in optical transmitter modules are, for example, constructed by using GaAs FETs or the like. In such semiconductor integrated circuits, increasing amplifier gain and band (bandwidth: operating frequency band) is demanded.
In the prior art, a source-coupled FET logic (SCFL) circuit having load devices, switching FETs (for example, GaAs FETS), and constant current source 5, is employed. In the prior art SCFL circuit, when changing the duty ratio of an output voltage, it has been practiced to apply a DC bias, and control its bias level, to accomplish the change.
For example, variations in the duty ratio of the output voltage are caused by process variations during the manufacture of the semiconductor integrated circuit and variations in the ambient temperature where the semiconductor integrated circuit is actually used. In the prior art, the desired duty ratio is obtained by externally controlling the bias level. In this way, an external device (for example, a laser diode in an optical transmitter module) controlled by the semiconductor integrated circuit, for example, can be operated in optimum condition.
Therefore, in the prior art semiconductor integrated circuit, the desired duty ratio is obtained by externally controlling the bias level and thereby compensating for the variation of the duty ratio of the output voltage. In this semiconductor integrated circuit, inductor peaking is sometimes used to increase the band of the circuit, but providing an inductor within a semiconductor chip (IC) increases chip area and hence the cost. To avoid this, a DC bias is applied and its bias level is controlled to vary the duty (duty ratio). In this case, however, the gain of the differential amplifier is almost reduced by one-half compared with the case when complementary signals are applied as the differential input signals.
A prior art semiconductor integrated circuit and optical transmitter module, and a problem associated with the prior art, will be described in detail later with reference to the drawings,